I. Field of the Invention
The present invention relates to a thermal printing head and, more particularly, to a thermal printing head which is integrally constituted by a heater array and a plurality of semiconductor devices each having a shift register.
II. Description of the Prior Art
The thermal printing system has been widely used as a system for obtaining a hard copy which is easily incorporated in a variety of equipment such as various types of printers and facsimile systems.
In the variety of applications described above, one of the important issues is to improve the printing speed of a thermal printing head used in a facsimile system or line printer.
In order to increase the head printing speed, (a) the pulse width of the applied voltage to each heater must be shortened; or (b) the number of heaters which are simultaneously operated must be increased. In method (a), it is difficult to apply a short power pulse having a pulse width of less then 1 mS from the point of view of the service life of the heaters (the heaters are required not to break down under an application of power pulses of, generally, 10.sup.7 to 10.sup.8 cycles) irrespective of the material and process of manufacture of the heater. In other words, in order to increase the printing speed of the line printer head, a number of heaters must be simultaneously effectively heated.
In a conventional diode matrix head integrally comprising a diode array and a heater array, generally, 16, 32 or 64 heaters are simultaneously heated.
In the head of the type described above, attempts have been made to simultaneously heat a number of heaters corresponding to an integer multiple of 16, 32 or 64 by rearranging the head terminal configuration. However, external driver circuits for driving the heaters are required in the same number as the heaters in order to use a diode matrix head, and lead wires for connecting the external driver circuits and the heaters are also required in the number of 16, 32 or (32.times.an integer); these factors preclude a compact printing unit.
A head is proposed in Electronics, Feb. 14, 1980, Page 191 in which a plurality of heaters are integrally arranged with a plurality of semiconductor devices each having a shift register for transferring an image signal corresponding to a pixel for driving a heater. In the head of this type, a semiconductor device has a circuit, which is slightly modified from the external driver circuit used in the conventional diode matrix head, integrally arranged with the heaters. Therefore, the head described in the above reference is neither new nor revolutionary, although it has advantages in that the head driver circuit has a simple construction and the number of lead wires for connecting the head and the external driver circuit is decreased, as compared with the conventional diode matrix head. It is noted that the above system described in Electronics features a shift register for transferring image signals and a plurality of transistors which are connected to the heaters.
The most important issues in forming the head of the type described above relate to the electrical connections between a number of heaters formed in an array and a plurality of semiconductor devices each having a shift register, and to the formation of the head terminals. The above issues greatly influence the outer appearance and shape of the head and the design of an apparatus using the head, and hence the manufacturing cost and reliability of the head.